The National Academies of Science has released an evaluation of the current …

Over the past several decades, there's been a stunning revolution in how we view the prospect of life on other planets. Starting with the Voyager missions in the 1970s, it became clear that the Solar System had a number of bodies that were geologically active. Evidence for oceans of liquid water and hints of Mars' watery past soon followed. Meanwhile, back on Earth, we began to discover that life could survive in some extremely harsh conditions: high radioactivity, near-boiling water, even under blood red ice in Antarctica.

These findings raised two prospects, one exciting, one worrisome. The Solar System could be much more hospitable to life than we ever thought, but if we sent probes looking for it, there was the chance we could transfer an Earth-origin extremophile to the new world, ruining everything. To that end, NASA established some rough guidelines for how to treat the risk of contamination on its deep space probes. Recently, the National Academies of Science was tasked with evaluating the protocols. The organization found them a bit lacking, based on a risk target that is "apparently arbitrary."

The existing standard for NASA missions to other worlds focuses on places like Mars and Jupiter's moon Europa, where the evidence of liquid water is extensive. The new report would add Saturn's Enceladus, which expels liquid water through visible geysers, and also Triton, the largest satellite of Neptune. This last body appears to be a Kuiper belt object captured by the planet (it orbits counter to Neptune's rotation). The Voyager flyby identified extensive surface remodeling and possible surface eruptions, suggesting the presence of liquids. Although this may be liquid nitrogen, we don't currently know enough about the moon to rule out the possibility of water.

If any of these worlds harbor either interesting prebiotic chemistry or even life, we'd obviously want to study it. And the surest possible way to eliminate that prospect is to introduce organisms from Earth as part of the process of accessing these environments. NASA has been considering how to avoid this since as far back as the Viking landers sent to Mars in the 1970s (which were baked at 111°C for a day and a half to kill anything off).

NASA's current policy dates back a bit over a decade, to another report by the National Research Council (the arm of the Academies responsible for this one). That was based on an earlier standard suggested by the Committee on Space Research (COSPAR) of the International Council for Science. COSPAR had suggested creating a list of the probabilities that Earth-based life would survive various events associated with a space probe, such as radiation exposure, extreme cold and heat, etc. If the probabilities were multiplied and came out to less than a one-in-10,000 chance of contamination, the risk was considered acceptable.

That one-in-10,000 chance, the new report finds, doesn't seem to have any solid basis. "Unfortunately, the historical literature does not record the rationale for COSPAR’s adoption of the 10-4 standard," the authors report. "Nor, in, fact has the committee been able to come up with its own quantitative rationale for this number." If the standard, as they say, is arbitrary, the process for evaluating the probability is seriously flawed. The process of multiplying probabilities together only works if those probabilities are independent, and there is no evidence that some of them are. For example, if cold resistant bacteria are more likely to be radiation resistant, it would throw the whole calculation off.

Given their dissatisfaction with the existing standards, the committee behind the new report has come up with a new one. They call for the use of a binary decision tree, in which a series of yes or no questions are considered: is there liquid water at the destination, is there some form of chemical energy to support life, etc. Strict attempts to control for contamination should only take place if the answers to most of these questions indicate there's a risk that it will happen.

The report also points out that there are a number of areas that we can research right here on Earth that would help us understand the risks better. Any organism that could find a home in the environments on other worlds will be, by necessity, very cold tolerant. But we haven't studied cold tolerant bacteria as well as we've looked at the ones that grow in hot environments. In particular, the report suggests that we look into whether the spores of cold tolerant organisms could survive some of the heat associated with decontamination and launch procedures, and whether they can form biofilms that aid their survival.

The authors also call for NASA to develop imaging technology that will actually let them scan spacecraft for microorganisms during assembly. This would help the launch team to make informed choices about decontamination, rather than relying on estimates or indirect measures.

Right now, it doesn't look like NASA will have the budget to go anywhere near most of the locations considered to be at risk of the contamination for many years. But the ESA is considering a Europa mission, and the report might help inform the decisions made by that agency. And, eventually, NASA will undoubtedly find a way to send probes out to look for new life.